The present invention relates to transponder devices, and more particularly to transponder devices or tags which can be programmed and which, when interrogated, provide a response in accordance with the program. The programming of the tag may involve the entry of data into a memory or may modify the mode of operation of the tag.
At present there are two commonly-used techniques for remotely entering data into tags. The first is to use a carrier signal as a reference clock to control internal circuitry within the tag. A second signal at a different frequency is then used to write or enter the data. Such arrangements are disclosed in U.S. Pat. No. 4,399,437 and U.S. Pat. No. 4,514,731. The alternative approach is to superimpose the data on the carrier by means of either phase or amplitude modulation. Where phase modulation is used, the carrier frequently is the means by which an internal reference oscillator within the tag is activated.
Both approaches have their limitations. With two separate input signals, it is necessary to incorporate two analogue input circuits and two input aerials. This both increases the cost of the tag and also the internal current requirements. In addition it involves a further burden of complexity on the control unit.
The use of a phase modulated approach with a single carrier requires that the system determines the sense of the aerials in the tag prior to writing any data. It also requires that the aerial orientation of the tag remains constant with respect to the aerials at the control equipment during the write phase. This involves a certain level of cost in additional circuitry and may impose some system limitations on its use.
Amplitude modulation overcomes the problem of orientation but suffers from limitations in dynamic range. For low frequency systems where the field strength decays in accordance with the inverse cube of range, adequate automatic gain control must be included within the input amplifier of the tag. This represents a difficult technical challenge and invariably limits the dynamic range which practically can be achieved.
EP-A-0289136 discloses a data transmission system in which the output of a controller resonant circuit is pulsed to change the logic level of a data signal read into the memory of a tag. However, clocking is determined according to a data transmission protocol, so the tag must have an internal timing circuit. Where appropriate, a new data bit is generated without a change in the output of the controller resonant circuit.